Butadiene is an important starting material for the production of high molecular weight polymers and is used extensively to form synthetic rubber including styrene-butadiene rubber, nitrile-butadiene rubber, buna-S rubber, and trans-polybutadiene rubber, and adiponitrile and styrene butadiene latex in paints. Butadiene is usually a by-product from steam cracking naphtha. However, the product butadiene regularly contains impurities that must be removed before the butadiene may be used as a starting material. The principal impurities are acetylenes including ethylacetylene, methylacetylene and vinylacetylene. Historically, two approaches have been used to remove the acetylenes: extractive distillation using a solvent to selectively absorb the acetylenes, or selective hydrogenation of the acetylenes. Examples of selective hydrogenation include U.S. Pat. Nos. 3,342,891, 3,541,178, 3,842,137, 3,898,298, 4,277,313, 4,469,907, 4,704,492, 5,414,170, and GB 2 040 995.
The patents U.S. Pat. Nos. 3,541,178, 3,842,137, 4,469,907, and U.S. Pat. No. 4,704,492 provide alternate methods of introducing hydrogen to the selective hydrogenation reactor. U.S. Pat. No. 3,342,891 teaches a process of fractionating the butadiene-containing stream into two portions with one portion enriched in acetylenes. Only that portion enriched in acetylenes is subjected to the selective hydrogenation. After selective hydrogenation the two portions are recombined. U.S. Pat. No. 5,414,170 discloses a process for selectively hydrogenating the acetylenes in an olefin plant process stream downstream of a front end depropanizer and upstream of further separation zones. U.S. Pat. No. 3,898,298 discloses selective hydrogenation of vinylacetylene using palladium on alumina catalysts at 35.degree. C. and 7 atmospheres to achieve mixed phase operation. GB 2 040 995 discloses admixing a recycle stream and a fresh C.sub.4 stream and hydrogenating the mixture, fractionating the product, feeding back an acetylenic stream and recovering butadiene. U.S. Pat. No. 4,277,313 discloses first selectively hydrogenating C.sub.4 -alkyne components and then using extractive distillation to separate a 1,3-butadiene-rich selective solvent extract phase.
As shown in the above references, the selective hydrogenation of acetylenes is generally performed using a catalytic composite. However, a drawback of catalytic hydrogenation is that some of the acetylenes will form polymeric by-products having from about 8 to about 36 carbon atoms; see Sarkany, A.; Weiss, A. H.; Szilagyi, T.; Sandor P.; Guczi L. Applied Catalysis 1984, 12, 373-379. These polymeric by-products must be removed before the butadiene can be successfully used as a starting material. One technique of removing the polymeric by-products is to distill the reactor effluent; see Abrevaya, H; Vora, B. V; Lentz, R. A., "Improved Butadiene Technology for Naphtha Cracking," Presented at the Fifth World Congress of Chemical Engineering, San Diego, July 1996. The present invention provides a process for the removal of polymeric by-products from an acetylene selective hydrogenation reactor product by employing low capital cost and low maintenance equipment and thereby reducing the overall cost of the process as compared to other selective hydrogenation processes. Specifically, the present invention eliminates the need for an expensive distillation column to remove polymeric by-products by using a cost efficient combination of a cooler, a low pressure flash drum, and a knockout drum.